This invention relates generally to the field of intraocular lenses (IOL) and, more particularly, to accommodative IOLs.
The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens.
When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. During this procedure, an opening is made in the anterior capsule and a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquifies or emulsifies the lens so that the lens may be aspirated out of the eye. The diseased lens, once removed, is replaced by an artificial lens.
In the natural lens, bifocality of distance and near vision is provided by a mechanism known as accommodation. The natural lens, early in life, is soft and contained within the capsular bag. The bag is suspended from the ciliary muscle by the zonules. Relaxation of the ciliary muscle tightens the zonules, and stretches the capsular bag. As a result, the natural lens tends to flatten. Tightening of the ciliary muscle relaxes the tension on the zonules, allowing the capsular bag and the natural lens to assume a more rounded shape. In this way, the natural lens can be focused alternatively on near and far objects.
As the lens ages, it becomes harder and is less able to change shape in reaction to the tightening of the ciliary muscle. This makes it harder for the lens to focus on near objects, a medical condition known as presbyopia. Presbyopia affects nearly all adults over the age of 45 or 50.
Prior to the present invention, when a cataract or other disease required the removal of the natural lens and replacement with an artificial IOL, the IOL was a monofocal lens, requiring that the patient use a pair of spectacles or contact lenses for near vision. Advanced Medical Optics has been selling a bifocal IOL, the ARRAY lens, for several years, but due to quality of issues, this lens has not been widely accepted.
Several designs for accommodative IOLs are being studied. For example, several designs manufactured by C&C Vision are currently undergoing clinical trials. See U.S. Pat. Nos. 6,197,059, 5,674,282, 5,496,366 and 5,476,514 (Cumming), the entire contents of which are incorporated herein by reference. The lens described in these patents is a single optic lens having flexible haptics that allows the optic to move forward and backward in reaction to movement of the ciliary muscle. Similar designs are described in U.S. Pat. No. 6,302,911 B1 (Hanna), U.S. Pat. Nos. 6,261,321 B1 and 6,241,777 B1 (both to Kellan), the entire contents of which are incorporated herein by reference. The amount of movement of the optic in these single-lens systems, however, may be insufficient to allow for a useful range of accommodation. In addition, as described in U.S. Pat. Nos. 6,197,059, 5,674,282, 5,496,366 and 5,476,514, the eye must be paralyzed for one to two weeks in order for capsular fibrosis to entrap the lens and thereby provide for a rigid association between the lens and the capsular bag. In addition, the commercial models of these lenses are made from a hydrogel or silicone material. Such materials are not inherently resistive to the formation of posterior capsule opacification (“PCO”). The only treatment for PCO is a capsulotomy using a Nd:YAG laser that vaporizes a portion of the posterior capsule. Such destruction of the posterior capsule may destroy the mechanism of accommodation of these lenses.
There have been some attempts to make a two-optic accommodative lens system. For example, U.S. Pat. No. 5,275,623 (Sarfarazi), WIPO Publication No. 00/66037 (Glick, et al.) and WO 01/34067 A1 (Bandhauer, et al), the entire contents of which are incorporated herein by reference, all disclose a two-optic lens system with one optic having a positive power and the other optic having a negative power. The optics are connected by a hinge mechanism that reacts to movement of the ciliary muscle to move the optics closer together or further apart, thereby providing accommodation. In order to provide this “zoom lens” effect, movement of the ciliary muscle must be adequately transmitted to the lens system through the haptics connecting the anterior lens to the posterior lens. Providing sufficient movement for a relatively broad range of accommodation through this mechanism has proven difficult.
Prior art accommodative two lens systems using a movable “zoom” lens have inherently limited movement. The maximum sensitivity or movement magnification α (a unitless ratio) is defined as the axial movement of the lens per unit zonule movement and is derived by the following equation:α=−B/Awhere B is the projected distance of the zonule length which is in the order of 1.0 to 2.0 mm; andA is the axial distance between the middle plane between the dual lens and the anterior surface of the anterior lens where the zonules terminate.
Practically speaking, because of the lens thickness and dual lens separation requirement, A cannot be less than ˜1 mm. Therefore, α cannot be larger than 2, which defines the limit of the known dual lens accommodative approaches. This limit is too low for the dual optics design to achieve the objective of creating the greater than 2.25 diopters of accommodative amplitude that patients need for normal accommodation, which ideally results in α greater than or equal to 4.
Therefore, a need continues to exist for a safe and stable dual lens accommodative intraocular lens that provides a relatively large amount of accommodative amplitude with minimal movement of the lenses.